Definición de common chord en inglés:

sustantivo

He introduced pupils first to the notes of the common chord (doh - me - soh), patterning the sounds with the voice and exercising the intervals involved until sound and symbol were firmly associated in the pupil's mind.

Where there is no figure under a note, the convention is that this denotes the most common chord, which Mr Protheroe describes as a root-position chord.